An emulsion is a dispersion of one liquid in another liquid and generally is in the form of a water-in-oil mixture having an aqueous or water phase dispersed within a substantially immiscible continuous oil phase. Water-in-oil (or oil in water) emulsions having a high ratio of dispersed aqueous phase to continuous oil phase are known in the art as High Internal Phase Emulsions, also referred to as “HIPE” or HIPEs. At relatively high dispersed aqueous phase to continuous oil phase ratios the continuous oil phase becomes essentially a thin film separating and coating the droplet-like structures of the internal, dispersed aqueous phase. The continuous oil phase of a water-in-oil HIPE generally comprises one or more polymerizable monomers. These monomers can be polymerized, forming a cellular structure, for example a foam, having a cell size distribution defined by the size distribution of the dispersed, aqueous phase droplets.
Polymerization of monomers in a HIPE usually involves the application of thermal energy or heat. The HIPE can be subjected to heat in an enclosed area, such as an oven; and the heat source can be varied, such as steam or direct application of heat through one or more heating elements. However there are several significant drawbacks to using heat alone for curing HIPEs. A HIPE usually has a large surface area and the unequal application of heat to such a large surface can cause defects in the resulting HIPE foam, such as dimpling, shrinkage, and edge curls. Further, whatever form the heat application takes such application is usually quite expensive in energy and monetary costs, as the heat being applied to a HIPE usually has a temperature ranging from around 50° C. to 150° C. An additional drawback to the use of heat alone to polymerize HIPEs is that an enclosed area, such as an oven, is required to keep the heat from escaping into the environment and reducing the amount of heat applied to the HIPE. This need for an oven further increases the cost of producing HIPE foams and takes up a large amount of physical space on the HIPE foam production line.
Attempts in the past to move away from the use of heat to polymerize HIPEs have used ultraviolet (UV) light to polymerize HIPEs into foams. However, these methods have had problems, such as a reliance on the use of polyelectrolytes in the HIPE which can lead to undesirable properties in the resulting HIPE foam, as polyectrolytes are trapped in the polymer backbone, whether through entanglements; polymerized in, via chain transfer reactions; or simply absorbed onto the surface of the HIPE foam struts. The polyelectrolytes thus incorporated into the HIPE foam change the physical properties of the foam, typically decreasing the strength and increasing friability of the resulting HIPE foam. Further, these methods have not been usable on thicker HIPEs and preferentially polymerize the monomers on the surface of the HIPE exposed to the source of the UV light, leaving the bulk of the HIPE unpolymerized. Accordingly, there is a need for a method of HIPE polymerization using UV light that overcomes the problems in the prior art.